A thermosetting resin such as an epoxy resin and a dicyanate resin has been widely used as a matrix of a carbon fiber-reinforced composite material, owing to its high thermostability, high chemical resistance to solvent or acid, high interfacial adhesive strength and wetting property against carbon fibers, and good manufacturing property. However, a carbon fiber-reinforced composite material employing the thermosetting resin as a matrix, due to its weak brittleness, has a shortcoming that separation may occur between plies of the matrix by exterior impact or fatigue("delamination"). Naturally, the delamination phenomenon has been the chief obstacles to use the carbon fiber-reinforced composite material as a primary structural material.
As a promising solution for the delamination phenomenon, a method for addition of rubber material or thermoplastic polymer to a thermosetting matrix resin to accomplish toughening of a matrix resin using an additive of uniformly dispersed form(about 0.1-5 .mu.m), which is formed by phase separation in a cure reaction, has been employed in the art(see: U.S. Pat. No. 3,472,730). The method are, however, proven to be less satisfactory in a sense that a toughness of the carbon fiber-reinforced composite material is less improved than that of a matrix resin itself.
Therefore, in order to solve the delamination phenomenon, an interleaved system that adhesive films or powder particles of a high toughness are added to a prepreg prior to cure reaction, has been developed in the art. For example, toughness of a carbon composite material against delamination can be improved by adding an elastomer of rubber material or thermoplastic resin between plies in a film form (see; U.S. Pat. No. 4,539,253; U.S. Pat. No. 4,604,319) or by adding a toughening agent between plies in a particle form(see: EP 0 274 899). However, the interleaved system has caused a weight penalty phenomenon that modulus and strength against volume of total laminate plates decrease, compared with a laminate plate having the same volume, since the adhesive films remain still between plies after cure reaction.
Recently, Murakami et al. added a toughening agent, polyimide in 10, 20 and 30 weight % to a carbon composite material which comprises an epoxy resin as a matrix, and measured a toughness against delamination(see: Murakami, A. et al., J. Adhesion, 39:227-242(1992)) . As a result, it was found that: a sea-island morphology is formed when polyimide of 10 weight % is added; and, a nodular morphology is formed when polyimide of 20 or more weight % is added. Also, once the nodular morphology is formed, a toughess against delamination is not increased even though increased amount of toughening agent is added.
In addition, Johnston et al., prepared a carbon composite material having a concentration gradient within a matrix by employing thermosetting polyimide and thermoplastic polyimide, and measured its mechanical property and toughness against delamination(see: 37th International SAMPE symposium, N. J. Johnston et al., March 9-12, pp.690-704(1992)). As a result, it was found that the concentration gradient of polyimide between layers of the carbon composite material allows to have a slightly increased chemical resistance and a toughness against delamination. However, it was revealed that the concentration gradient of polyimide in a single phase does not give a satisfactory toughening effect.